In recent years, there is movement of replacing a vinyl chloride-based resin with other material because of, for example, a problem of environmental influence. Among numerous substitute materials, a polyester resin is an influential material in terms of its physical nature, environmental suitability, adhesive property, cost and the like.
Among polyester resins, a crystalline polyester resin such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polylactic acid and the like is molded into heat resistant parts by injection molding, into films, or sheets by extrusion molding, into drink bottles by blow molding, and into fibers by melt spinning.
On the other hand, since an amorphous polyester resin is excellent in transparency, gloss, impact resistance, and stress whitening suppression and, further, is easy in molding processibility as compared with a crystalline polyester, a representative of which is PET, the resin is molded into monolayer or multilayer sheets, lamination films, or shrinkable films by extrusion molding, into miscellaneous goods, stationery, or cosmetic containers by injection molding, or into price rails, or IC tubes by profile molding.
However, in these polyester resins, during melt molding, thermal degradation or hydrolysis occurs, leading to reduction in a molecular weight. In particular, when polyester resins are not sufficiently dried, since this reduction in a molecular weight is remarkable, there is a problem that mechanical physical property of products is considerably deteriorated.
Further, in the case of direct blow molding, since this reduction in a molecular weight makes drawdown phenomenon remarkable, and nonuniformity of wall thickness and burr become large, the ratio of the article that meets standard specifications is reduced. At the same time, since the parison forming state is not stabilized, continuous production stability is also reduced. In addition, since mechanical physical property such as impact resistance of a molded article is reduced, a bottle strength at filling contents into a bottle container is deficient, a bottle is bursted at falling, and contents are flown out.
In recant years, a PET bottle is used at a large amount in various refreshing drink fields such as mineral water and the like. On the other hand, accompanying therewith, a problem of disposal of the PET bottle is given an attention, and PET bottle scrap treatment has become a social problem. Under such the situation, recycling of used PET bottles is being studied intensively.
For recycling PET bottles, used PET bottles are recovered, ground, and washed to obtain a reproduced PET flake, and there are the case where a product is molded using this as a raw material, and the case where this is once pelletized by melt extrusion, and a product is molded from this pellet via melt molding.
Since this reproduced PET flake is reduced in a molecular weight by heat, contains a large amount of water by a washing step, and is worse as for biting property on a screw of a molding machine due to the flaky form, the flake is difficult to mold and, only by the reproduced PET flake, only a brittle product having unstable quality is obtained. In addition, since PET usually is hydrolyzed easily and, when molded in the undried state, hydrolysis is promoted, and a molecular weight is reduced at melting, mechanical physical property such as impact resistance is considerably reduced. Therefore, a molded article using the reproduced PET flake does not satisfy performance required in usual plastic products in many cases.
Previously, as study on improvement in impact resistance, a method of melt molding by blending an impact resistance modifier into a polyester has been known. An effective impact resistance modifier for a polyester is known in Japanese Patent Publication No. 59-28223 gazette. In the gazette, a rubber-like polymer obtained by grafting or copolymerizing with a compound having a functional group capable of chemically reacting with a polyester is used. Particularly effective impact resistance modifier is an olefin-based or a styrene block polymer-based polymer. In addition, Japanese Patent Laying-Open No. 11-269360 shows that, by blending a polytetrafluoroethylene-containing powder consisting of polytetrafluoroethylene and an organic polymer into a polyester resin, drawdown at molding is improved. Japanese Patent Laying-Open No. 2001-2903 shows that, by blending a multilayered structure particle consisting of a rubber-based component into a polyester resin, impact resistance of a polyester resin molded article produced from a reproduced PET flake is improved. Japanese Patent Laying-Open No. 2001-114995 shows that, by blending polypropylene, and an ethylene-based or styrene-based polymer modified with unsaturated carboxylic acid into a polyester resin, impact resistance is improved. Japanese Patent Laying-Open Nos. 2002-146167 and 2002-249649 show that, by blending a thickener (rubber-based resin) and a high-molecularizing agent (ethylene-based or styrene-based elastomer) into a polyester resin, impact resistance is improved.
In case that the impact resistance modifier and the polyester resin are kneaded as mentioned above, reduction in a molecular weight due to re-extrusion progresses further. So improvement in impact resistance by addition of a small amount of an impact resistance modifier is not sufficient. In addition, since this is addition of a heterogeneous polymer, transparency originally possessed by a polyester resin is inhibited due to a difference in refractive indices. Therefore, under such the circumstances, impact resistance is improved at the cost of transparency.
Japanese Patent Laying-Open No. 2000-355657 shows that, by blending a polymer composed of a vinyl polymer in a thermoplastic resin, particularly, a polyester resin, flowability is improved while maintaining transparency. In this case, there remains a problem that, even when applied to a polyester resin, transparency can be maintained, but it is, difficult to suppress reduction in a molecular weight, and impact resistance can not be improved.
In blow extrusion, an agent with reactive functional groups for increasing melt strength, whose weight average molecular weight is 1 million to 4 millions, is proposed as a reactive melt strength improving agent (Japanese Patent No. 3237913). However, in this kind of agent for increasing melt strength, since a vinyl-based aromatic modifier having an extremely high molecular weight must be dispersed in a polyester resin, it is necessary to thoroughly adjust its blending amount and processing condition. For example, in order to attain an objective melt strength, processing condition is limited to a narrow range and, when departed from this condition, a melt strength is greatly varied. In addition, probably due to poor compatibility between an agent for increasing melt strength and a polyester resin, there is problem that even at slight bending at application of this molded article, voids are generated, and this part is whitened.
As described above, a modifier which inhibits reduction in physical property due to reduction in a molecular weight and, at the same time, can maintain transparency while impact resistance is improved, for a polyester resin, has been sought, but such the modifier has not been proposed yet.